The present invention relates to a structure for the crankcase of an internal combustion engine which is equipped with a balancer shaft that rotates parallel to a crankshaft.
In a reciprocating piston type internal combustion engine, generally speaking, there are generated vibrations of a first order which result from the reciprocations of the piston. In order to eliminate these first order vibrations, there has been adopted the concept of laying a balancer shaft parallel to the crankshaft and turning it at the same speed as, but in the opposite direction to, the crankshaft and integrate in the balancer shaft a balance weight having a suitable shape such as a sector shape. However, since an internal combustion engine equipped with such balancer shaft has its crankshaft and balancer shaft revolving in the opposite directions to each other, tensile and compressive loads are repeatedly applied to the crankcase walls which are interposed between the bearing portions of the crankshaft and the bearing portions of the balancer shaft. These repeated tensile and compressive loads may cause cracks in the crankcase walls. In order to cope with this, the prior art tends to make these wall portions thicker. However, not only do the thicker walls necessarily enlarge the size of the internal combustion engine, but also there is a limit to the wall thickness in case the compressive and tensile loads are high. If excessively thick walls are formed, there arises the risk that cavities are liable to be formed in the crankcase (which is made of an aluminum die casting).
The present invention has been conceived in view of the background thus far described and has as an object to provide a crankcase for an internal combustion engine having a balancer shaft, which can withstand repeated high compressive and tensile loads without making the walls of the crankcase thicker, and which is so constructed as to be free from any formation of cavities.